A filter is an apparatus for passing only signals of specific frequency band out of all input frequency signals, and is implemented in a variety of forms. The band passing frequency of an RF filter is determined by the inductance components and capacitance components of the filter, and the work of adjusting filter characteristics such as band passing frequency and bandwidth of the filter is called tuning.
FIG. 1 is a drawing illustrating the structure of a general RF cavity filter of the related art.
Referring to FIG. 1, a general RF cavity filter of the related art comprises a housing 100, an input connector 102, an output connector 104, a cover 106, multiple cavities 108, and a resonator 110.
An RF filter is an apparatus for passing only signals of specific frequency band out of all input frequency signals, and is implemented in a variety of forms.
The inside of a filter has multiple walls, which define cavities 108 in which individual resonators are placed. The cover is equipped with coupling holes and tuning bolts 112 for coupling the housing 100 and the cover 106.
The tuning bolts 112 are coupled to the cover 106, penetrating into the inside of the housing. The tuning bolts 112 are placed on the cover 106 in correspondence with the resonators or designated positions within the cavities.
RF signals are input by the input connector 102 and output to the output connector 104, and proceed through a coupling window formed in each of the cavities. Resonance phenomenon of RF signals occurs by means of the cavities 108 and resonators 110, and RF signals are filtered by means of the resonance phenomenon.
In a filter of the related art such as that illustrated in FIG. 1, tuning for frequency and bandwidth is achieved by tuning bolts.
FIG. 2 is a drawing illustrating a cross-section of a cavity in an RF cavity filter of the related art.
Referring to FIG. 2, the tuning bolt 112 penetrates the cover 106 and is positioned above the resonator. The cover 106 has holes for tuning bolts to go through, and the holes have screw threads formed therein.
The tuning bolts 112 can be turned along the screw threads formed in the holes, so that the distance between it and the resonator may be adjusted, and tuning is achieved by changing the distance between the resonator 110 and the tuning bolt 112. The tuning bolt 112 may be turned manually, or by a separate tuning machine for turning the tuning bolt.
The tuning bolt should be connected electrically with a ground, and is conventionally connected through the cover and screw thread of a grounded filter. Here, there exist micro-gaps between screw threads, and oxidized material can form in such gaps. Oxidized material that forms in the gaps is a major cause of a spark phenomenon and Passive Inter-Modulation Distortion (PIMD) when an RF cavity filter is used in high power, and such spark phenomenon and PIMD are factors that impede stable grounding of tuning bolts.
To prevent such phenomena, U.S. Pat. No. 4,775,847 proposed a structure that connects a separate grounding member between the underside of the filter cover and the tuning bolt, so that the electric current passes through the grounding member and flows to the filter cover.
FIG. 3 is a drawing illustrating an example of a tuning bolt grounding structure using a grounding member to prevent the spark phenomenon and PIMD at high power.
Referring to FIG. 3, in the tuning bolt grounding structure using a grounding part proposed by U.S. Pat. No. 4,775,847, a grounding member 302 is additionally included, and the grounding member 302 provides another path for the electric current to flow from the tuning bolt through the grounding part to the filter cover, besides the path for the electric current to flow from the tuning bolt 300 through the screw threads to the filter cover. The grounding member is coupled to the filter cover by means of rivets.
When a foreign material occurs in the gaps between screw-threads on the inside of a hole, the electrical current will flow in the path passing through the grounding member, due to a difference in impedance, and as a result, a spark phenomenon by the foreign material can be prevented.
However, such tuning bolt grounding structure according to the related art has the problem of being vulnerable to PIMD. PIMD refers to the phenomenon whereby two or more signal frequencies cause interference in passive components, thus causing unwanted parasitic signals.
Causes of PIMD in RF components can be largely divided into two categories: contact nonlinearity and material nonlinearity. Among the factors causing contact nonlinearity are: junction capacitance due to a thin layer of oxidation between conductors; tunnel effect due to semiconductor action between conductors in metal contact; micro-discharge due to empty spaces and minute cracks between metals; nonlinearity related to dust and metal particles on the metal surfaces; and constriction resistance occurring in metallic bonds. Among the factors causing material linearity are: hysteresis effect of nickel, iron, cobalt, etc.; internal Schottkey effect; and thermal heating due to limited conductivity in a conductor.
The conventional tuning bolt grounding structure illustrated in FIG. 3 is one whereby contact nonlinearity increases as contact area between metals increases and especially as the grounding member and the filter cover are coupled by means of rivets, and is one that is inevitably vulnerable to PIMD.
Also, since the grounding member and tuning bolt are in linear contact, the contact area is small and cannot have sufficient grounding effect, and has the problem of being vulnerable to the external environmental factors such as vibration.
In recent times, elimination of PIMD is one of the main challenges in improving the performance of mobile communication systems, and requires a tuning bolt grounding structure that can suppress the occurrence of PIMD.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.